In the production of filamentary and/or web materials such as, for example, films, papers and the like, it is important that any variations in certain properties of such materials be detected not only for purposes of detecting defects for quality control purposes and rejection of defective material but also for monitoring of properties on-line to effect control of manufacturing processes. Detection of any variations in the properties being monitored should be made as they occur and with minimum delay before large quantities of unacceptable material are produced. In order to be most effective, such detection should be made by means of continuous, on-line monitoring of the properties being measured.
The use of microwave devices is known in the art for the measurement of certain properties, such as thickness and dielectric anisotropy, in papers, films and similar materials. A device for measuring properties of a material by microwave resonance techniques is disclosed in U.S. Pat. No. 3,458,808--Agdur, which describes a microwave resonant cavity having at least two resonant frequency peaks and which is driven by an external microwave generator which sweeps a range of frequencies. A selected property of the test material, such as thickness or moisture content, is determined by measuring the time interval between resonant frequency peaks which is affected by changes in the property being measured. Other such devices driven by external sweep oscillators are known in the art. Such devices have a basic disadvantage in that they must sweep the range of frequencies in order to take each measurement and can thus make only one measurement for each sweep cycle. They therefore are unable to detect changes or defects which may be present, for example, in short lengths of moving materials such as moving web or filamentary materials or in small amounts of material on a moving web or belt where the time interval for detection may be too short relative to the time required for a complete sweep.
In a paper by M. Tiuri and P. Liimatainen entitled "A Microwave Method for Measurement of Fiber Orientation in Paper", published in the Journal of Microwave Power, 10(2), 1975, a method of measuring the dielectric anisotropy of paper is disclosed in which a dual resonant mode microwave cavity having orthogonally oriented resonant modes is used. To the extent that the dielectric constant of the paper is different in one direction than in another, the resonant frequency of one of the orthogonal modes will be affected to a greater degree than the other. The degree and direction of anisotropy of the dielectric constant and hence the fiber orientation of the paper is thus determined. However, the system was not used to measure the properties of the material nor was there any suggestion of any such possible use. Other methods of measuring magnetic or dielectric anisotropy at microwave frequencies have been suggested in the prior art.
In a paper entitled "Microwave Moisture Meters for the Paper and Pulp Industry" by Kjell Lindberg and Ulf Ternstrom, published in Measurement and Control, Vol. 3, March, 1970, various techniques are disclosed for measuring the moisture content of paper web using changes in the resonant frequency of a microwave cavity caused by the wet material. It is suggested in that paper that a resonator having an infinite number of discrete resonant frequencies, each belonging to a certain mode of oscillation and having a certain field configuration, can be utilized. It is further suggested that a measuring mode can be selected which has a field configuration such that the wet material affects the resonant frequency belonging to that mode significantly, and that a reference mode can be selected with a field configuration such that the resonant frequency is substantially independent of the wet material. There is no suggestion as to how this might be accomplished. Further, the suggestion does not deal with the type of cavity to be used and assumes that a discrete and different resonant frequency for each mode would be necessary.
Other means for measuring properties of materials include capacitive and infrared sensing devices and techniques. Capacitometers are sensitive to electrostatic charges which build up on certain materials such as yarn or film and which create sources of noise in the measurements. When used in a yarn line, most capacitometers have the electric field oriented perpendicular to the yarn line and thus can not be used for measurements on flat yarns because they yield different readings dependent on fiber orientation in the field. Infrared sensors for measuring certain properties such as moisture content are very slow and can require a number of yards of moving material to obtain a measurement, so that localized or short length variations are either averaged out or not detected at all.